A method and corresponding apparatus for performing optical time domain reflectometry is disclosed in EP-A-No. 0 269 448. According to the known optical time domain reflectometer (OTDR), sequences of light pulses are injected into an optical fiber and the backscattered light pulses are correlated with the injected sequences of light pulses to generate a representation of the amplitude of the backscattered signal as a function of the time elapsed since the injection of the sequence or as a function of the distance from the input end of the fiber. The pulse sequences injected into the fiber are, for example, complementary pseudorandom sequences such as sequences in accordance with Golay codes. A pair of complementary sequences A and B have the property that the autocorrelation product of A has sidelobes which are complementary to the sidelobes of the autocorrelation product of B; i.e., sidelobes which are located at the same point in the time spectrum and which have amplitudes of equal magnitudes, respectively, but of opposite sign. Consequently, if the correlation products corresponding to the complementary sequences are superposed, the sidelobes cancel, and there remains only signals indicative of actual reflections in the fiber. Ideally, the superposed correlation products of the injected complementary pulse sequences with the signals reflected from distinct discontinuities in the fiber, such as fractures, are a single sharp peak or delta function. The use of complementary pulse sequences provides a better signal-to-noise ratio than conventional reflectometry wherein only a single pulse is injected into the fiber and the reflection of this particular pulse is detected.
In practice, the sidelobe cancellation is not perfect because of the non-ideal characteristics of the signal processing circuitry. For example, saturation of electronic components, such as a receiver or analog-to-digital converter, due to strong power levels of the reflected signal can lead to non-linearities which destroy the complementary nature of the correlation products. As a consequence, correlation sidelobes may appear on the measured backscatter curve that are large enough to distort or even mask a large reflection peak.